1. Field of the Invention
The invention is directed to methods for the preparation of stable heteropoly molybdate catalysts and use of certain of the obtained catalysts particularly in the conversion of C.sub.3 and C.sub.4 olefins to their corresponding unsaturated aldehydes and unsaturated acids.
2. Description of the Prior Art
The preparation of acrylic acid from propylene and the preparation of methacrylic acid from isobutylene are generally carried out in two stages under independently selected conditions for each stage and most often employing different catalysts selected for each stage. In the first stage the olefin is catalytically oxidized to the aldehyde with the formation of possibly a small amount of the corresponding C.sub.3 or C.sub.4 unsaturated acid. The obtained aldehyde is converted to the acid in the second stage generally in the presence of a different catalyst. Among the better known commercial processes, the production of acrolein is effected by catalytic oxidation of propylene with oxygen over supported CuO catalyst at about 350.degree. C.; or by air oxidation of propylene over BiO.sub.3 /MoO.sub.3 catalyst at 300.degree.-360.degree. C. The oxidation of the acrolein formed by the foregoing or other processes is carried out in a second catalytic reactor generally at a temperature of about 250.degree. C.
Numerous different catalysts have been proposed in the prior patented art for the conversion of acrolein and methacrolein to the corresponding unsaturated acids, more generally comprising metal molybdates and phosphomolybdates combined with various metal cations. Typical among such are U.S. Pat. Nos. 3,326,817; 3,865,873; 3,875,220; 3,882,047; 3,925,464; 3,976,888; 3,998,876; 4,000,088; 4,001,316; 4,017,423; 4,025,565; 4,035,262; 4,042,533; 4,042,625; 4,045,478, 4,051,179; 4,070,397; 4,072,708; 4,075,123; 4,075,124; and 4,075,244.
As an example of such prior art methods is that described in U.S. Pat. No. 3,965,163. As therein described, a solution of antimony trichloride in hydrochloric acid is combined with phosphomolybdic acid, and tungsten trioxide is then added to the obtained solution. The resulting mixture when dried and calcined forms a catalyst having the empirical formula EQU Sb.sub.1 Mo.sub.12 W.sub.1 P.sub.1 O.sub.41.5
U.S. Pat. No. 4,034,008 discloses the oxidation of alpha, beta unsaturated monoolefins with molecular oxygen to produce the unsaturated acid as well as the unsaturated aldehyde. The reaction is carried out in the presence of steam at about 350.degree.-450.degree. C. over catalyst comprising a major portion of molybdena associated with oxides of bismuth, iron, silica, nickel or cobalt, antimony or ruthenium and optionally containing chloride ion. At best only about 1 to 2 parts by weight of acrylic acid are obtained along with about 13 to 15 parts acrolein or about 1 to 2.65 parts acrylic acid with 16 to 19 parts acrolein.